The present invention relates to a semiconductor device having a double integral analog-digital conversion circuit, and furthermore, an electronic control device using the analog-digital conversion circuit, and relates to a technology effective when being applied to an electronic control device for controlling an apparatus such as a power train while monitoring a resistance in a measurement sensor, for example, in an in-vehicle system.
In order to monitor a resistance of a measurement sensor, the double integral analog-digital conversion circuit can be used from its feature of being capable of high precision conversion in a simple circuit. As a representative of the double integral analog-digital conversion circuit, for example, as described in Japanese Unexamined Patent Application Publication No. 2012-39273, there is a circuit for finding a digital output by integrating an input voltage and a reference voltage whose polarity is opposite to the value two times in succession with a Miller integrating circuit. For example, an input voltage Vin can be found by counting a time elapsed when a voltage of an integrator varies from an output voltage Vo obtained by integrating the input voltage Vin for a specified time to a voltage of 0 V obtained by integrating the reference voltage Vref. When assuming the double integral analog-digital conversion circuit of FIG. 9, a conversion operation is performed by integrating the analog input voltage Vin with an integrator with and subsequently integrating the reference voltage Vref with the integrator again. V1 during a period when the analog input voltage Vin is integrated by the Miller integrating circuit becomes as shown by Formula 1. Incidentally, parameters in the following Formula 1 to Formula 3 are as follows. V1: output voltage Vo of the integrator at the time of integrating the input voltage Vin; V2: output voltage Vo of the integrator at the time of integrating the reference voltage Vref; t1: the specified time; N2: pulse count at time t1; t2: integral time of the reference voltage Vref; and n: pulse count at time t2 (digital output).
            [              Formula        ⁢                                  ⁢        1            ]        ⁢                                                          V            1                    =                                                    -                                  Vin                  RC                                            ⁢                              t                1                                      =                                          Vin                RC                            ⁢              NT                                                            (          1          )                    
V2 during a period when the reference voltage Vref is integrated by the integrator becomes as shown by Formula 2.
            [              Formula        ⁢                                  ⁢        2            ]        ⁢                                                          V            2                    =                                                    V                1                            -                                                1                  RC                                ⁢                                  (                                      -                    Vref                                    )                                ⁢                                  t                  2                                                      =                                          V                1                            -                                                1                  RC                                ⁢                                  (                                      -                    Vref                                    )                                ⁢                nT                                                                          (          2          )                    
In Formula 2, it shall be integrated until V2 becomes V2=0 and a relationship of Formula 3 is obtained by substituting Formula 1 into this.
            [              Formula        ⁢                                  ⁢        3            ]        ⁢                                              n          =                                    Vin              Vref                        ·            N                                                (          3          )                    
As is clear from Formula 3, the input voltage Vin is converted into the pulse count (digital output) of n.
For example, when intending to monitor a resistance of a thermistor of a temperature sensor, a reference resistance can be coupled in series to the thermistor serving as the measurement resistance and its divided voltage can be used as the input voltage. If the reference resistance Rpu and the measurement resistance Rm are coupled in series between the reference voltages Vp, Vn and a voltage of their binding node is used as the input voltage Vin, the measurement resistance Rm can be found by Formula 4.
            [              Formula        ⁢                                  ⁢        4            ]        ⁢                                              Rm          =                                                    Vn                -                Vin                                            Vin                -                Vp                                      ·            Rpu                                                (          4          )                    